1. Technical Field
The present invention relates to an arrangement for an internal combustion engine of the turbocompound type and that includes an exhaust system for ducting the engine's exhaust gases. A supercharger turbine drives a compressor for the engine's combustion air and an exhaust turbine is also included that is located in the exhaust system downstream of the supercharger turbine for extracting residual energy from the exhaust flow via transmission to the crankshaft of the internal combustion engine. The exhaust system also includes an exhaust braking throttle located downstream of the exhaust turbine.
2. Background Art
In a turbocompound engine (TC engine), power is transmitted from the power turbine of the TC unit, via a gear mechanism, down to the engine's crankshaft. This power is obtained by extracting the residual energy that remains in the engine's exhaust gases after having passed through the turbo compressor for compressing the engine's charging air.
For engine braking, it is normal for an exhaust brake to be used. For a TC engine, the exhaust brake consists of a suitable arrangement, normally a throttle valve that can throttle the exhaust flow, and it is placed downstream of the TC unit. When the valve is closed and the fuel injection ceases, power is transmitted instead from the crankshaft via a gear to the TC unit's power turbine. This power helps to increase the braking effect as it is an energy loss, but which is positive from a braking perspective. A problem that can arise, however, is that a valve that has been closed downstream of the TC unit will increase the density of the air in which the TC unit's power turbine is operating. This, of course, assists the braking, but it also gives rise to increased thermal and mechanical stresses. These increased stresses will depend upon the engine speed and will increase with increased engine speed. In order that the TC unit or other components do not break, they must be dimensioned for the increased stresses. This can lead to the construction being unnecessarily expensive, as it is made more complicated and as expensive heat-resistant material must be used.
With an exhaust brake, the braking effect increases for a given engine speed when the back pressure after the turbine increases. In order to obtain best braking function, as high as possible back pressure is desired. For a TC engine, this is particularly difficult, as the stresses that were mentioned above arise as a result of the back pressure increasing. For a given back pressure, the stresses also increase when the engine speed increases. In order that components do not break, they must be constructed so that they can withstand the stresses that arise at the maximal permitted engine brake speed. Alternatively, a lower back pressure can be selected. A lower back pressure can be presumed still to give acceptable braking performance at high engine speeds, but at low engine speeds the braking effect is commensurately low. Thus, in order to obtain a good braking effect at low engine speeds, high back pressure is required. This in turn leads to large forces at high engine speeds, or alternatively using low back pressures at a high engine speed and obtaining a poor braking effect at low engine speeds.